:?:{4 Tiie American (Teohxjitst. Doccmbcr, is95 
Bamboo, Wis., that have been appealed to, are more reasonably 
explainable on the hypothesis of a progressive subsidence of 
the region and submergence beneath the ocean, incident to an 
epoch of voluniinovis igneous ejection. Furthermore, this sub- 
sidence is not a hypothesis; it is proved. In that case thei-e 
may have been no atmospheric erosion-interval, but, after local 
fracturing of the ocean's bed and the out])ouring of lava, sed- 
imentation was resumed and resulted in alternating igneous 
and clastic strata. Toward the end of the epoch of disturb- 
ance fractures occurred in some places without the issue of 
lava, while in other places lava was poured out in vast 
amounts. The former may be seen on the south side of the 
lake, as at Keweenaw point, and the latter in the Xijugon 
region. 
It might be remarked, in addition to the objections already 
urged against a great erosion-interval at this horizon, that, if 
the Lower ('ambrian is entirely wanting in the Lake Sujierior 
region, the top of the pre-Cambrian there presents an anomaly. 
The top of the [)re-( 'ambrian is usually a very tirm and even a 
crystalline rock. Its great age necessitates this — i. e., if the 
Cambrian be not limited at the Olenellus zone. Here, how- 
ever, the hyi)othesis of the Wisconsin survey, adopted by the 
U. S. Geological Survey, reveals the top of the pre-Caiubrian 
as an erosible red sandstone with an upper limit that is wholly 
unknown, sonu^times tilted and sometimes horizontal, its con- 
tact with the horizontal Cambrian above iici-er IkivIikj lii-m 
seen. At the same time, the base of this erosible sandstone is 
equally elusive, since it vanishes in a nuiss of conformable 
eruptives, whose appearance is so fresh that they have been 
classed as Mesozoic by several geologists. On so slender a 
thread does this important hypothesis hang. 
In regard to the separation of the Keweenawan system as 
defined by Irving, into three parts. Lower. Middle and Ui)per 
Cambrian, each marked by its eruptive rocks and character- 
ized by its own elastics, there seems to be not only much evi- 
dence, but even a necessity for such differentiation. No 
student of the geology of the Lake Sui)erior region can avoid 
the conviction of something anomalous and bizarre in tiic 
composition and structural features of the Keweenawan as it 
has been described and mappetl. Instead of the individual 
